Aromatic hydrocarbons, particularly benzene, toluene, ethylbenzene and xylenes, are important commodity chemicals in the petrochemical industry. Currently, aromatics are most frequently produced from petroleum-based feedstocks by a variety of processes, including catalytic reforming and catalytic cracking. However, there is a growing need to find alternative sources of aromatic hydrocarbons.
One possible alternative source of aromatic hydrocarbons is methane, which is the major constituent of natural gas and biogas. World reserves of natural gas are constantly being upgraded and more natural gas is currently being discovered than oil. Because of the problems associated with transportation of large volumes of natural gas, most of the natural gas produced along with oil, particularly at remote places, is flared and wasted. Hence the conversion of alkanes contained in natural gas directly to higher hydrocarbons, such as aromatics, is an attractive method of upgrading natural gas, providing the attendant technical difficulties can be overcome.
A large majority of the processes currently proposed for converting methane to liquid hydrocarbons involve initial conversion of the methane to synthesis gas, a blend of H2 and CO. However, production of synthesis gas is capital and energy intensive and hence routes that do not require synthesis gas generation are preferred.
A number of alternative processes have been proposed for directly converting methane to higher hydrocarbons. One such process involves catalytic oxidative coupling of methane to olefins followed by the catalytic conversion of the olefins to liquid hydrocarbons, including aromatic hydrocarbons. For example, U.S. Pat. No. 5,336,825 discloses a two-step process for the oxidative conversion of methane to gasoline range hydrocarbons comprising aromatic hydrocarbons. In the first step, methane is converted to ethylene and minor amounts of C3 and C4 olefins in the presence of free oxygen using a rare earth metal promoted alkaline earth metal oxide catalyst at a temperature between 500° C. and 1000° C. The ethylene and higher olefins formed in the first step are then converted to gasoline range liquid hydrocarbons over an acidic solid catalyst containing a high silica pentasil zeolite.
However, oxidative coupling methods suffer from the problems that they involve highly exothermic and potentially hazardous methane combustion reactions and they generate large quantities of environmentally sensitive carbon oxides.
A potentially attractive route for upgrading methane directly into higher hydrocarbons, particularly ethylene, benzene and naphthalene, is dehydroaromatization or reductive coupling. This process typically involves contacting the methane with a catalyst comprising a metal supported on a zeolite, such as ZSM-5, at high temperature, such as 600° C. to 1000° C.
For example, U.S. Pat. No. 4,727,206 discloses a process for producing liquids rich in aromatic hydrocarbons by contacting methane at a temperature between 600° C. and 800° C. in the absence of oxygen with a catalyst composition comprising an aluminosilicate having a silica to alumina molar ratio of at least 5:1, said aluminosilicate being loaded with (i) gallium or a compound thereof and (ii) a metal or a compound thereof from Group VIIB of the Periodic Table.
In addition, U.S. Pat. No. 5,026,937 discloses a process for the aromatization of methane which comprises the steps of passing a feed stream, which comprises over 0.5 mol % hydrogen and 50 mol % methane, into a reaction zone having at least one bed of solid catalyst comprising ZSM-5, gallium and phosphorus-containing alumina at conversion conditions which include a temperature of 550° C. to 750° C., a pressure less than 10 atmospheres absolute (1000 kPaa) and a gas hourly space velocity of 400 to 7,500 hr−1.
Moreover, U.S. Pat. Nos. 6,239,057 and 6,426,442 disclose a process for producing higher carbon number hydrocarbons, e.g., benzene, from low carbon number hydrocarbons, such as methane, by contacting the latter with a catalyst comprising a porous support, such as ZSM-5, which has dispersed thereon rhenium and a promoter metal such as iron, cobalt, vanadium, manganese, molybdenum, tungsten or a mixture thereof. After impregnation of the support with the rhenium and promoter metal, the catalyst is activated by treatment with hydrogen and/or methane at a temperature of about 100° C. to about 800° C. for a time of about 0.5 hr. to about 100 hr. The addition of CO or CO2 to the methane feed is said to increase the yield of benzene and the stability of the catalyst.
Further in our U.S. Published Patent Application No. 2007/0260098, we have described a process for converting methane to higher hydrocarbons including aromatic hydrocarbons, the process comprising contacting a feed containing methane with a dehydrocyclization catalyst, conveniently molybdenum, tungsten and/or rhenium or a compound thereof on ZSM-5 or an aluminum oxide, under conditions effective to convert said methane to aromatic hydrocarbons and produce a first effluent stream comprising aromatic hydrocarbons and hydrogen, wherein said first effluent stream comprises at least 5 wt % more aromatic rings than said feed; and reacting at least part of the hydrogen from said first effluent stream with an oxygen-containing species to produce a second effluent stream having a reduced hydrogen content compared with said first effluent stream.
Despite these proposals, the successful application of reductive coupling to produce aromatics on a commercial scale requires the solution of a number of serious technical challenges. For example, the reductive coupling process is both endothermic and thermodynamically limited. Thus the reaction is generally conducted at high temperatures, such as in excess of 700° C., but the cooling effect caused by the reaction lowers the reaction temperature sufficiently to greatly reduce the reaction rate and total thermodynamic conversion if significant make-up heat is not provided to the process. In addition, the process tends to produce carbon and other non-volatile materials, collectively referred to as “coke”, that accumulate on the catalyst resulting in reduced activity and potentially undesirable selectivity shifts, as well as loss of valuable feedstock. The inventor has therefore now concluded that, in an embodiment, the process is best operated in a moving or fluidized bed reaction system which also allows for rapid and repeated transfer of the catalyst between a reaction cycle and a regeneration cycle as well as enabling heat supply by means of hot solids carrying heat into the reactor.
As a result of consideration of these and other constraints on the reductive coupling reaction, the inventor has also now determined that in embodiments the optimum catalyst employed must meet an exacting specification including some or all of the following properties:                (a) high hardness to resist mechanical attrition;        (b) high mechanical strength to resist attrition due to thermal shock;        (c) high thermal conductivity to reduce aging during reheating;        (d) high heat capacity to reduce the catalysts circulation rate required to provide a given heat input to the system;        (e) high particle density to allow higher gas velocities in the reactor;        (f) absence of components that directly or indirectly promote coke formation; and        (g) high accessibility to catalytically active sites to provide improved resistance to catalyst deactivation by coke accumulation.        
In view of these constraints, there is a continuing need to find new catalyst systems that are tailored to the exacting duty required by the reductive coupling reaction.
In accordance to the invention, it has now been found that an improved catalyst for the dehydroaromatization of methane can be produced by using, as a catalyst support, zeolite crystals grown in the pores of a porous refractory material, such as foamed silicon carbide, and depositing the catalytically active component(s) on the zeolite.
U.S. Pat. No. 7,179,764 discloses a catalyst composite comprising a zeolite deposited on a support, wherein the support comprises silicon carbide (SiC) with a specific BET surface area of at least 5 m2/g, wherein the support comprises a silicon carbide foam. However, the catalyst composite is produced by creating a surface layer of silica between 1 and 10 nm thick measured by XPS on the support by calcination; putting the support in contact with a previously set gel that is capable of forming the zeolite, and conducting a hydrothermal synthesis to form the zeolite.
U.S. Published Patent Application No. 2005/0056568 discloses the use of supported catalysts comprising at least one metal or metallic compound of a metal from group VI and/or group VIII deposited on a support essentially constituted by β-silicon carbide in a process for selective hydrodesulphurization of an olefinic hydrocarbon feed that is substantially free of polynuclear aromatics and metals. The process is said to allow deep desulphurization of catalytically cracked gasoline cuts with very limited saturation of olefins and thus a minimum loss of octane number.
U.S. Published Patent Application No. 2006/0121239, the entire disclosure of which is incorporated herein by reference, discloses a silicon carbide based porous material containing silicon carbide particles as an aggregate and metallic silicon as a bonding material and having a number of pores formed by them, characterized in that it has an oxide phase in at least a part of the pores, and the oxide phase contains respective oxides of silicon, aluminum and an alkaline earth metal and contains substantially no alkaline earth metal silicate crystal phase.
U.S. Published Patent Application No. 2009/0029103, the entire disclosure of which is incorporated herein by reference, discloses a silicon carbide-based porous article comprising silicon carbide particles as an aggregate, metallic silicon and an aggregate derived from siliceous inorganic particles to form pores through volume shrinkage by heat treatment, wherein the porosity is 45 to 70%, and the average pore diameter is 10 to 20 μm. The article is produced by a method comprising; adding inorganic particles to form pores through volume shrinkage by heat treatment to a raw-material mixture containing silicon carbide particles and metallic silicon, then forming into an intended shape, calcinating and firing the resultant green body, forming pores through volume shrinkage of the inorganic particles by heat treatment, and the shrunk inorganic particles being present as an aggregate in the porous article.
International Patent Publication No. WO2009/034268, the entire disclosure of which is incorporated herein by reference, discloses a composite, useful as a catalyst substrate, comprising a layer of porous alumina deposited on a rigid substrate made of beta-SiC. The alumina layer may include catalytically active phases, in particular phases that do not properly bind onto the non-treated beta-SiC, such as silver particles.
German Patent Publication No. 102007031537, the entire disclosure of which is incorporated herein by reference, discloses a process for producing a porous silicon carbide composite having active functional centers. The process comprises forming a microemulsion containing a chemical compound or element for formation of the active functional center, a non ionic surfactant and an organic silicon-containing compound. The microemulsion is subjected to thermal treatment at 1100-1500° C. in an inert atmosphere to a form of silicon carbide containing functional centers and the residual carbon is removed by heating in an oxidizing atmosphere at 1000° C.